Control circuit for an electrolytic cell

ABSTRACT

A control circuit for an electrolytic cell in which a positive feedback loop is utilized to increase the voltage across and the current through the electrolyte as the conductivity of the electrolyte increases.

Aug- 7, 1973 H. MANDROIAN 3,751,355

CONTROL CIRCUIT FOR AN ELECTROLYTIC CELL Filed Feb. 8, 1971 lmmmm p mmmm )o @gj E v q 2 b a VO I -r-rof/mfs y United States Patent O U.S. Cl. 204-228 14 Claims ABSTRACT F THE DISCLOSURE A control circuit for an electrolytic cell in which a positive feedback loop is utilized to increase the voltage across and the current through the electrolyte as the conductivity of the electrolyte increases.

BACKGROUND OF THE INVENTION (l) Field of the invention This invention relates to control circuitry for electrolytic cells.

(2) Description of the prior art It is well known in the art to recover products from solutions by an electrolytic process. This well known process, simply stated, involves immersing a pair of electrodes in an electrolytic solution containing the product to be recovered and impressing across the electrodes a voltage of suicient magnitude to cause electrolytic deposition of the desired product. One use of the electrolytic recovery process, for example, is in recovering silver from used photographic hypo solutions by plating the silver on a stainless steel cathode. In order to eiiiciently recover silver from such photographic hypo solutions, it was found desirable to be able to regulate the current through such solution as the concentration of the recoverable silver ions therein varied. More specifically, as the concentration of the silver ions in the solution increased, the resistance of the electrolyte decreased and in order to recover silver (or any other metal in the solution) at a higher rate, it was found desirable that the current through the solution also be increased. One method for doing this is shown in U.S. Pat. No. 3,450,622, to O. J. Cothran, which discloses a circuit by which the voltage is held constant during the electrolytic process and the current is permitted to vary in proportion to the conductance of the electrolytic solution. While the utilization of such a control circuit allowed the silver to be recovered in a more eiiicient manner, it has been found that a change of current that is merely in proportion to the conductance of the electrolytic solution does not yield optimum recovery rates, and that optimum results are obtainable only by varying the voltage across and the current through the electrolytic solution as the resistance of the electrolytic solution varies.

OBJECTS AND SUMMARY OF THE INVENTION The principal object of the present invention is to provide a new and improved control system for an electrolytic cell.

A more specific object of the invention is to provide a control system for apparatus for recovering products from electrolytic solutions, which control system maximizes the recovery rate of such products from the electrolytic solution.

Another object of the invention is to provide a control circuit for electrolytic recovery apparatus in which the voltage across and the current through the electrolytic solution varies as the resistance of the electrolytic solution varies.

A further object of the invention is to provide a control circuit for electrolytic recovery apparatus in which lCe the current through the electrolytic solution varies in a non-linear manner as the resistance of the electrolytic solution varies.

Another object of the present invention is to provide a control circuit for electrolytic recovery apparatus which senses the resistance of the electrolytic solution and through the use of feedback controls the voltage across and the current through the solution.

In accordance with one embodiment of the present invention, a control circuit is provided which has a full wave controlled rectifier coupled to a saturable core reactor. The condition of the saturable core reactor is controlled by the use of negative voltage feedback taken across the output leads of the control circuit and positive current feedback taken from a sensing circuit coupled to the negative terminal of a recovery cell and the negative output lead of the control circuit. The negative voltage feedback causes the current to increase in a linear manner proportional to the change in conductance of the electrolyte. The positive current feedback, however, causes the voltage across the electrolyte to increase as the resistance of the electrolyte decreases and to thus cause the current through the electrolyte to increase in a non-linear manner as the resistance of the electrolyte decreases.

The novel features of the invention are set forth with particularity in the appended claims. The invention will best be understood from the following description when read in conjunction With the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGSl FIG. l is an illustration of a preferred circuit of the invention;

FIG. 2 is la graph of current versus voltage illustrating the operation of the invention;

FIG. 3 is a graph of current versus resistance illustrating the operation of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. l a schematic diagram of the control circuit of the present invention is illustrated. Power source means 10 impresses 115 volts A.C. across transformers T1 and T2 through switch 12 and fuse 14. Neon bulb 16 serves to indicate the OFF-ON condition of the circuit. The secondary of the transformer T1 is attached to the pump motor (not illustrated) of a recirculating electrolytic recovery cell such as shown in Cothran. The secondary of transformer T2 steps the 115 volts A.C. down to 6.3 volts A.C. which is applied to the anodes of controlled rectiers CR1 and CR2. The positive output terminal of the control circuit is coupled to the cathodes of the controlled rectiers CR1 and CRZ while the negative output terminal of the control circuit is coupled to a center tap in the secondary of the transformer T2.

The output of the secondary of transformer T2 is also coupled through diodes 118 and 20 to a saturable core reactor 22 which controls the gate current to the controlled rectiers CR1 and CR2 and thus the firing time of such rectifiers. The condition of the saturable reactor 22 is controlled partially by coils 24 and 26 coupled to diodes 18 and 20 and partially by coils 28 and 30. Coils 24 and 26 are coupled to the gates of CR1 and CR2 and load 27. Coil 28 is coupled by lead 32, resistor 34, and potentiometer 36 to the output terminals of the control circuit and has passing through it a current which is dependent upon the voltage across such output terminals. Coil 30 is coupled by lead 38, non-linear resistor 40, whose resistance increases with increasing current, and potentiometer 42 to sensing terminal 44 which is coupled to the negative terminal 46 of the electrolytic recovery cell 48 and has passing through it a feedback current which is dependent upon the voltage drop across lead 50 between terminal 46 and the negative output terminal of the control circuit, i.e. upon the amount of current flowing between the anode and the cathode of the electrolytic recovery cell 48.

In one embodiment the following components were used:

Diodes 18, 20=1N4997 load 27 :100 ohms resistor 34:3'90 ohms potentiometer 36:200 ohms potentiometer 42:20-30 ohms In operation, when the control circuit is activated by power source means 10, a voltage Vo of the order of .5 to 1.5 volts, as shown in FIG. 2, appears across the positive and negative output terminals after a fraction of a second of operation when such terminals and the sensing terminal 44 are in an open circuit condition. This voltage appears after the reactor 22 has been saturated by the current flowing through coils 24, 26, coming from the secondary of the transformer T2, and the controlled rectiiiers have been made conductive by the application of a gate voltage from such coils. Voltage Vo is Va fully rectified D.C. voltage. T'he particular value of voltage Vo is a result of the circuit characteristics and can be adjusted by varying the potentiometer 36 and thus the current through coil 28 which determines the amount the reactor 22 is reset. In general, then, the open circuit condition of the reactor 22, and thus the initial operating voltage, is a function of the current impressed on the coil 28 via lead 32 and the input current from the secondary of the transformer T1 impressed upon the coils 24 and 26.

When the positive and negative terminals of the control circuit are coupled to the positive and negative terminals of the electrolytic recovery cell 48, a current I flows through the electrolyte between the anode and the cathode of the electrolytic recovery cell 48. The magnitude of the current I is dependent on the conductivity of the electrolyte and the value of the voltage across the recovery cell. As stated previously, the prior art control circuits held the voltage across the recovery cell constant with the result that the current that flowed through the prior art recovery cell was proportional to the conductance of the electrolyte. This condition would also exist in the present circuit if the sensing terminal 44 were not connected to the negative terminal 46 of the electrolytic recovery cell 48. The plot of voltage Versus current and current versus resistance R of the cell in such a case would appear as shown in curves a of FIGS. 2 and 3, with the voltage being' held constant and the current increasing in inverse proportion to the decrease in resistance R of the electrolytic solution.

When, however, as in the present invention, sensing terminal 44 is connected to the negative terminal 46 of the electrolytic recovery cell 48, a current ows through coil 30 which decreases the effectiveness of the current flowing through coil 28, and thus causes a lesser resetting of the cores in the reactor 22. Consequently, as the resistance R of the electrolyte decreases, there is not only a corresponding initial increase in current I but also an increase in voltage V across the output terminals of the control circuit. This increase in voltage causes an additional increase in current through the electrolyte with the result that the nal increase in current for a particular decrease in resistance R is greater than it would have been had the sense terminal 44 not been connected. The relationship of such increasing voltage and current with respect to one another as the resistance R varies, and the increase of current with respect to a decrease in resistance R is shown in curves b of FIGS. 2 and 3. The amount of positive current feedback through lead 38 is also controlled by potentiometer 42 which can be set at a preselected value. As can be seen in a comparison of curves a and b of FIGS. 2 and 3, the connection of sensing terminal 44 causes current I to increase linearly with voltage V and to have an increasingly greater magnitude Ias the value of resistance R decreases due to the incremental increase in voltage V caused by the positive current feedback through lead 38.

Since the value of the voltage Vo can be adjusted by varying the potentiometer 36, it is often quite desirable to reduce the voltage V0 so that voltage V is below the value of the constant voltage (usually a compromise voltage) shown in curve a of FIG. 2 for high values of resistance and above the value of such constant voltage for low values of resistance. Such a voltage V, plotted against current I, is shown in curve b' of FIG. 2. This adjustment of voltage allows a maximum recovery rate at low values of resistance where, for example, the silver concentration is high and at the same time prevents sulfiding in the electrolyte (due to excessive voltage and current) at high values of resistance where, for example, the silver concentration is low. As can be seen in curve b of FIG. 3, due to such voltage adjustment, the current I is lower than the current shown in curve a of FIG. 3 accompanying such constant voltage or high values of resistant R and higher than such current for low value of resistance R, thus maximizing the recovery rate and at the same time preventing sulfiding. As previously noted, the amount of positive feedback can be varied considerably by adjusting potentiometer 42. This in turn changes the slopes of curves b and b of FIGS. 2 and 3 and allows slow or rapid changes of voltage and current with respect to resistance depending on the type of electrolyte being used.

Although a particular embodiment of the control circuit has been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art. The saturable core reactor, for example, may be replaced by a phase shifter which would be controlled by the levels of the feedback voltages from the positive and negative feedback loops, the voltage from the positive feedback loop serving to advance, and theA voltage from the negative loop to delay, the firing time of the controlled rectifiers. In addition, other circuits may be devised using feedback to increase both the voltage and the current as the resistance decreases. It is apparent, moreover, that such circuit can be used to control plating processes as well as recovery processes. Consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.

What is claimed is:

1. A control circuit for an electrolytic cell having an anode, a cathode, and an electrolyte therein comprising:

power source means; coupling means coupling said power source means and said anode and said cathode for applying a voltage across and a current through said electrolyte; and

sensing means for sensing the resistance of said electro lytic cell and coupled to said coupling means for causing said voltage to decrease or increase as the resistance of said electrolytic cell increases or decreases.

2. The circuit of claim 1 wherein said sensing means includes non-linear resistance means.

3. The circuit of claim 2 wherein the resistance of said non-linear resistance means increases with increasing current passing therethrough.

4. The circuit of claim 1 wherein said sensing means is coupled to one terminal of said electrolytic cell and the corresponding output lead of said coupling means and controls said voltage by means of positive current feed back dependent on the voltage drop between said terminal and said output lead.

5. The circuit of claim 1 wherein said sensing means causes the current across said electrolyte to vary in a nonlinear manner as the resistance of said electrolytic cell varies.

6. A control circuit for an electrolytic cell having an anode, a cathode, and an electrolyte therein comprising power source means; coupling means coupling said power source means and said anode and said cathode for applying a voltage across and a current through said electrolyte; and

sensing means including said anode and said cathode for sensing the resistance of said electrolytic cell under the actual operating conditions of said electrolytic cell and coupled to said coupling means for controlling said voltage with respect to the resistance of said electrolytic cell as the resistance of said electrolytic cell varies.

7. The circuit of claim 6 wherein said anode and said cathode are utilized in both the sensing and plating functions of said electrolytic cell.

8. The circuit of claim 6 wherein the sensing and plating functions occur at the same voltage level.

9. The circuit of claim 6 wherein said cell resistance includes the anode-electrolyte resistance and the cathodeelectrolyte resistance.

10. The circuit of claim 6 wherein said sensing means causes said voltage to decrease or increase as the resistance of said electrolytic cell increases or decreases.

11. The circuit of claim 6 wherein said sensing means includes non-linear resistance means.

12. The circuit of claim 11 wherein the resistance of said non-linear resistance means increases with increasing current passing therethrough.

13. The circuit of claim 6 wherein said sensing means is coupled to one terminal of said electrolytic cell and the corresponding output lead of said coupling means and controls said voltage by means of positive current feedback dependent on the voltage drop between said terminal and said output lead.

14. The circuit of claim 6 wherein said sensing means causes the current across said electrolyte to vary in a nonlinear -manner as the resistance of said electrolytic cell varies.

References Cited UNITED STATES PATENTS 3,551,318 12/1970 Suook et al 204-228 3,524,805 8/ 1970 Engelman 204-228 FOREIGN PATENTS 206,676 6/1968 U.S.S.R. 204-228 HOWARD S. WILLIAMS, Primary Examiner D. R. VALENTINE, Assistant Examiner U.S. Cl. X.R. 

